The X-Ray Timing Explorer (XTE) satellite includes a science payload including a proportional counter assembly, an all-sky monitor and a high-energy X-ray timing experiment. The XTE also includes gimbal-mounted high-gain antennas (HGA's) for communication with the ground station and memory to store information to be transmitted to the ground station so that when the XTE is out of range of the ground station, the information can be stored until the XTE comes back into range. The HGA's send and receive information in a conventional mode known as OMNI mode.
An overview of the XTE is shown in FIG. 1. Satellite 100 includes satellite body 102, solar panels 104 and high-gain antennas 106A, 106B mounted by means of gimbals 108A, 108B.
The X-Ray Timing Explorer (XTE) uses a data system developed by Goddard Space Flight Center known as the "Flight Data System" (FDS). This data system implements packet telemetry and command standards; its internal architecture is based on a fiber optic serial bus known as MIL-STD-1773. FDS uses solid state. recorders to improve system performance and reliability. The hardware uses an Intel 80386 microprocessor, although, of course, other microprocessors could be substituted as needed, and the software provides a distributed modular architecture that is readily extendible to meet new mission requirements.
The FDS receives commands from the ground and delivers them to on-board subsystems. It collects engineering and science data for telemetry transmission to the ground, records data for playback when out of ground contact, and provides autonomous spacecraft operation. It provides real-time control in a distributed multiprocessing environment, packet data communication services and packet data telemetry acquisition.
The design of the hardware is based extensively on the hardware developed for the Small Explorer Data System (SEDS) and the TRMM Spacecraft Data System. SEDS was used on the SAMPEX spacecraft that was launched in July 1992, and it continues to perform successfully.
The software includes an operating system layer, a communication layer and an application layer. The operating system layer uses a commercial multitasking operating system kernel and supports task scheduling and basic inter-task communications. The communication layer includes a software bus and a 1773 scheduler. The software bus provides a standard software interface for sending or receiving data packets, which are in the documented CCSDS format which is known to those skilled in the art, thus allowing the exchange of data among tasks. The 1773 scheduler performs input/output (I/O) operations between FDS software tasks to other components of the satellite along the fiber optic serial bus. The application layer performs such functions as command management, telemetry data acquisition, data storage, telemetry output, spacecraft time maintenance and distribution, spacecraft health and safety management, telemetry data monitoring, antenna management and instrument support. The software further includes a system management function for allowing operators at the ground station to access all layers and more specifically to manage, reconfigure and reload software at all layers.
As shown in FIG. 2, the SDS hardware includes three 1773 buses: attitude control system (ACS) bus 202 connected to attitude control systems such as gimbal control system 208, spacecraft control (S/C) bus 204 connected to critical spacecraft components 210, and instrument bus 206 connected to instruments 212. ACS bus 202 is connected to attitude control processor 214, which includes an 80386 chip or the like. S/C bus 204 is connected to attitude control processor 214, uplink interface 216 and spacecraft control (S/C) processor 218, which also includes an 80386 chip or the like, to allow control of spacecraft components 210 under either ground commands or commands generated on board the satellite Instrument bus 206 is attached to S/C processor 218. Also included are downlink interface 220, memory 222 and transponder 224.
It would be desirable to allow the XTE to communicate with the ground station via a tracking and data relay system satellite (TDRSS). However, such communication is not possible without a way of tracking the relative positions of the XTE and the TDRSS, which are in motion relative to each other as well as to the ground station, and of controlling antennas on the XTE to make contact with the TDRSS.